1. Field of the Invention
The present invention relates generally to an inspection system, and in particular to a inspection system for detecting explosives and contraband using a nuclear quadrupole resonance (NQR) tube array coil.
2. Discussion of the Related Art
Presently, the detection of contraband substances may be accomplished using a variety of different inspection and detection systems implementing technologies such as nuclear magnetic resonance (NMR), thermal neutron analysis (TNA), X-ray, and nuclear quadrupole resonance. Although each of these technologies may be useful in many applications, they all have limitations of varying degrees. For example, one drawback of NMR is that it requires relatively large magnets, which are somewhat expensive and could damage magnetically recorded material.
Although TNA based systems can detect nitrogen in most forms, these systems are susceptible to false alarms triggered by nitrogen-rich nylon and wool, and other innocuous items. X-ray screening, commonly used in airports, does not have the same overall limitations as TNA. However, many X-ray screening systems are unable to automatically alert an operator to the presence of explosives or drugs since these types of systems simply provide images that the operator must interpret.
In prior NQR testing systems, a sample is placed within a radio frequency (RF) coil, and is typically irradiated with pulses or sequences of pulses of electromagnetic radiation having a frequency which is at or very close to one or more resonance frequencies of the quadrupolar nuclei in a substance which is to be detected. If the substance is present, the irradiating energy will generate a precessing magnetization which can induce voltage signals in a detection coil adjacent the sample at the resonance frequency, and which can be detected as a free induction decay during a decay period after each pulse, or as an echo after two or more pulses.
Conventional NQR systems typically implement a sheet coil that is shaped and configured to define a cavity for receiving a sample to be screened. While such sheet coil configurations have enjoyed considerable success in many respects, the sheet configuration of the RF coil inhibits or prevents the use of additional or auxiliary sensor systems. NQR systems comprising RF coils that facilitate cooperation with auxiliary sensor systems have not previously been known.